In order to use hydrophobic materials such as thermoplastic resins, elastomers, sealants, paints, adhesives and the like in various fields, it has been requested to provide these materials with both water-absorbency and water retention properties. As methods for providing these materials with water-absorbency, hitherto, various proposals have been made and there are known, for example, a method for enhancing hydrophilic properties by chemically treating the surface of a hydrophobic material and a method for blending a water-absorbent substance with a hydrophobic material. Among them, since the method using a water-absorbent substance can be relatively readily carried out and high water-absorbency can be obtained, many investigations thereof have been done. Particularly, the method using water-absorbent resin particles has widely been employed in practice.
However, there is a problem that a water-absorbent resin has a high water absorption capacity and, therefore, the resin particles on the surface of a hydrophobic material readily absorb water and swell to cause removal thereof from the surface of the hydrophobic material due to falling off of the swollen particles, which results in a rough surface of the material. And, in the extreme case, falling off of the resin particles gradually extends to the inside of the material and, sometimes, water-absorbency of the hydrophobic material is completely lost. In order to solve such a problem, Japanese Patent Kokai No. 56-33032 discloses a method comprising dispersing finely divided particles of a water-absorbent resin having a particle size of 100.mu. or finer in a flexible resin or rubber uniformly. However, it is insufficient to solve the above problem and, when the product is soaked in a water for a long period of time, falling off of the water-absorbent resin particles is excessive.
In Japanese Patent Kokai No. 62-174259, a silicone rubber is provided with water-absorbency by using a water-absorbent resin and, in order to avoid the problem of falling off of the water-absorbent resin, a specific kind of silicone rubber is chosen.
Japanese Patent Kokai No. 61-200142 discloses the use of an ethylene-acrylic acid copolymer. However, there is no disclosure about the above problem of falling off of the water-absorbent resin particles at all.
Under these circumstances, the present inventors have studied intensively to solve the above problem. As the result, it has been found that a water absorption capacity of the water-absorbent resin itself has great influence on this falling off phenomenon upon absorption of water and the surface appearance of the above hydrophobic materials. That is, it has been found that, in general, as a water absorption capacity of a water-absorbent resin becomes higher, an amount of the water-absorbent resin removed from a hydrophobic material due to falling off becomes larger and, in the case of soaking the material in water for a long period of time, the amount of the resin removed becomes excessive, which results in loss of water-absorbency and water retention properties.
Nevertheless, in each of the above conventional techniques, water-absorbent resins having a high capacities of several hundreds to 1000-fold are used as they are, and it has been considered that water-absorbent resins having low water absorption capacities are undesirable because of their inferior water-absorbency and water retention properties.
On the contrary, according to the present inventors, it has been found that a water retention material with water-absorbency having a good surface appearance with minimum removal of a water-absorbent resin upon absorption of water can be obtained by using a water-absorbent resin in which its water absorption capacity has been adjusted to such a low capacity as 2 to 50-fold.
In fact, even by using a water-absorbent resin having a low water absorption capacity, the resulting water retention material with water-absorbency sufficiently functions as waterstopping materials, caulking materials, dew condensation-proof materials and the like and, in practice, use of a water-absorbent resin having high water absorption capacity is not necessarily required.
On the contrary, it has been unexpectedly found that uniform dispersion of a water-absorbent resin in a hydrophobic material can be readily obtained by using a large amount of a water-absorbent resin having a low water absorption capacity rather than by using a small amount of that having a high water absorption capacity.